1. Field of the Invention
This invention relates to a method of manufacturing a solid polymer electrolyte, especially one applicable for an electrochemical element such as a primary battery, secondary battery, condenser, or an electrochromic display device.
2. Prior Art
In the past, as the electrolyte for a primary battery, secondary battery, condenser, or an electrochromic display device, liquid state electrolyte has been used.
However, a liquid electrolyte is apt to cause devices in which it is involved to have leaking defects and to be of low reliability for long time usage.
On the other hand, a solid electrolyte has no such defects. With the application of the solid electrolyte, electrochemical devices as referred to above become smaller and lighter, and highly reliable without any anxiety for electrolyte leakage, and their fabrication simpler. Accordingly, research and development of solid electrolytes have been actively carried out.
The solid polymer electrolyte on which research and development has been made is divided into three materials; inorganic, organic and composite materials.
As for the inorganic material, silver iodide, Li.sub.2 Ti.sub.3 O.sub.7, .beta.-alumina, RbAg.sub.4 I.sub.5, and wolfram phosphate etc. are known. However, many of them are difficult to be formed into an arbitrary shape such as a film, and, for obtaining sufficient ion-conductivity, need to be raised to a higher temperature than room temperature. Besides, raw materials therefor are expensive.
Composite materials thereof with resin, proposed to overcome such shortcomings in film-forming, involve instability of ion conduction, since the boundary between the inorganic materials is apt to be broken due to outer stress.
As materials to remedy such shortcomings described above, some organic materials have been given attention and have been the object of research.
The organic material consists of a polymer and an electrolyte, the former making a matrix and the latter working as a carrier. Since the high ion-conductivity of the material, which is a complex of polyethyleneoxide (hereinafter referred as PEO) and alkali metal salt, was reported, researches on such solid polymer electrolyte as PEO, polypropyleneoxide, polyethyleneimine, polyepichlorohydrin, or polyphosphazene have been actively carried out. Such solid polymer electrolytes of organic materials are, compared with that of inorganic materials, of light weight, of high energy-density, of good mechanical flexibility, and of suitable adaptation to film-making process. Research works to obtain solid polymer electrolytes with high ion-conductivity, while keeping such inherent favorable characteristics, are intensively carried out.
Among the ideas proposed in the past, we find one to use the above-described normal-chain polymer as a solid electrolyte, which exploits the phenomena that ions dissociated in a matrix polymer becomes, by associating with the oxygen in the polymer, a solvent, and is, by application of electric field, diffuse-transported by repeating association and dissociation. In this case, ions are transported, changing the locations of chains of polymers by thermal motion of polymers. Accordingly, polymers with low glass-transition temperature are preferrable and sufficient. However, in these normal-chain polymers, crystalization takes place under room temperature resulting in lowered ion-conductivity.
To realize high ion-conductivity in solid polymer electrolyte under room temperature, existence of an amorphous region is necessary. For that purpose, to cross-link polyoxyalkyleneglycerin with alkylenediisocyanate (Japanese patent application open laying No. Sho 63-55811) or to cross-link with tolylenediisocyanate has been proposed.
Also, a cross-link resin made by copolymerization of polyoxyalkylene having active double bonds at both ends and polymethoxyoxyalkylene having a double bond at one end has been proposed.
Thus, to obtain a high ion-conductivity electrolyte by causing a polymer matrix of polyethylene oxide structure to involve nonaqueous electrolyte containing an inorganic salt, was suggested. In such material, however, there still remained shortcoming because they were liquid.
Among oxycompounds such as ethylenecarbonate or polypropylenecarbonate, vinylenecarbonate is found to have double bond. Research to produce ion-conductive solid polymer with the polymer of vinylenecarbonate has been made since the middle of 1980s (Solid State Ionics, 25 37-40 (1987), Polymer, 30 504-508 (1989). According to the research, polyvinylenecarbonate is, by being mixed with crown ether and inorganic salt, formed to film, whose ion conductivity is 3.times.10.sup.-4 S cm.sup.-1.
Also, it is proposed to mix an isocyanate cross-linked complex of polyethyleneoxide, polyvinylenecarbonate or poly.beta.-propiolactone, and inorganic salt, and cast into film-form (Japanese patent application open-laying No, Hei 2-295070). With the isocyanate as used for conventional methods being highly reactive, it is difficult to realize a reproducible cross-linked state, unless water content and activity of isocyanate itself are controlled at the preparation stage. Also, when the urethane cross-linked complex is used in a battery, active hydrogen atoms in urethane bond are decomposed and cut off, resulting in instability of the electrolyte.
Also, copolymers of the compounds having reactive double bonding had the difficulty in obtaining high ion-conductivity due to restriction of motion of an oxyalkylene chain by the polymerized main chain.
Further, these solid-state polyethyleneoxides, poorly dissolving inorganic salts, and not attaining sufficient numbers of dissociated ions, could not realize high ion-conductivity.